Diffusion cloud chamber



March 28, 1961 R. E. FEARON DIFFUSION CLOUD CHAMBER Filed March 28, 1958DIFFUSION CLOUD CHAMBER Robert E. Fear-on, Tulsa, iOkla., assigner toElectro Chemical Laboratories Corporation, Tulsa, Okla., 'a

corporation of Delaware Filed Mar. 28,v 1958, Ser. No. 724,743

12 Claims. (Cl. Z50-83) The present invention relates to apparatus fordemonstrating visually the tracks of ionized particles and moreparticularly to diffusion cloud chambers.

Droplet precipitating cloud chambers, starting with the so-called Cloudapparatus of C. T. R. Wilson, operate on the same general principle,namely, that a supersaturated vapor will condense into liquid dropspreferentially at places whereL there are charged particles, such ascharged atoms or molecules. The charged particles act as centers or'nuclei for the drops which are formed. Electrically charged atoms ormolecules are produced in profusion, in pairs of equal numbers ofpositive and negative particles along the path of any ionizing particle,

such as a rapidly moving free electron, any alpha particle, any chargedmeson, any high speed proton or deutron,

any fission particle, or any recoiling nucleus set in mo' tion by beinghit by a neutron. These are examples of the many circumstances underwhich tracks will appear in a supersaturated atmosphere. Such tracks areespecially instructive, for the reason that, under suitable conditionspractically every charged atom or molecule becomes the location of adrop very quickly after it is generated, and a count of the drops undergood magnification will determine how many ion-pairs there were in thetrack, or in the portion of it which is observed. From this, some. veryfundamental things may be learned, such as, for example, how manyelectron volts of particle energy were released in the observed portionof the track. l Also, in view of the fact that the density of thedroplets is eX- peoted Ito increase as .the ionizing particle slows down(in terms of the number of drops perunit path length) itis oftenpossible to look at a track and tell which way the particle moved inpassing over the trajectory it followed in producing the track,l i.e.,the sense of direction of the motion of the primary particle along thetrack.

As a practical matter, charged atoms and molecules are not the onlypoints in space where a droplet can be initiated in a supersaturatedatmosphere. Dust particles, and even peculiar molecules of certainoffensive contaminating substances can fool the vapor molecules andproduce false centers of droplet formation, not corresponding in any waywith nuclear radiation. In general, unless there is a continuous sourceof some offending cloud seeding material, the particles and molecules ofthis sort shortly become exhausted by beingcarried down with thedroplets they acquire, and are thus swept out of the gas` and vaporfilled space. cloud chambers, such purging takes quite a long time. Forvarious reasons, smallness of structure favors quick purging.

Another practical matter in the design andl operation of a cloud chamberis supersaturation, and the degree thereof. Too much supersaturationproduces completely nonselective droplet formation, and makes dropletseverywhere all the time, in complete disregard of nuclear tracks. In.thepexpansion chamber of C. T, R. Wilson, this was-arranged bycarefulchoice of the initial Ytempera- In many of the larger e ture, andcontrol of the degree of expansion (adiabatic) of the originally watervapor saturated space.

In the case of a diffusion cloud chamber the control of the degree ofsupersaturation is accomplished by choice of temperature of the vaporsource, the gradient of temj perature, in terms of thenumber of degreesper unit distance downward in the region of vertical gradation` oftemperature, and the rate of diffusion of the Vapor in question inwhatever gas is chos'en for the atmosphere..

An insufiicient rate of supply of vapor, or an insufficient verticalgradient of temperature, results in too'.

small a degree of supersaturation. 'I'he evidence of this condition isfailure of they droplets to form at all on the tracks, which are noteffective for nucleation unless the excess of vapor pressure aboveequilibrium is sufiicient.

With too much supersaturation, too many tracks appear.

and they will, in general, have no relevancy to nuclear` radiation.

The conventional cloud chambers commercially available are generally oflarge size so that the walls will be remote from the major portion ofthe active volumev of the chamber. Thus relief of supersaturation in anyzone by diffusion of vapor molecules to the walls and condensationthereon are not a major problem in the operation of such a device. Alsoin large size cloud chambers g of vconventional design, the supply ofvapor, e.g., methanol, may be controlled by separate devices such as avariablel Also, in the commercial, models of large size, there is butlittle trouble maintain-I ing exactly and in an orderly manner thedesired ver-J tical gradient of temperature in the system, for therea-,-v

heat source for the` methanol.

son that the disturbing effect of the walls, temperaturewise, is remote.Such commercial models of cloud chambers also provide temperatureregulation systems to main.- tain their cold sourcesat constant,determined low temperatures.

The principal object of the invention has been the.Y provision of anovel and improved diffusion cloud chamt ber. Y

tion, such as that of about 200 electron volts.

Still another object of the invention has been the provision of a no-veland improved diffusion cloud chamber having a low recovery time afterradiation overload.,l

A further object of the invention has been the proviy sion of a noveland improved diffusion cloud of simple and economical construction whichaffords good,

reproducible performance.

A feature of the invention has been the provision of. a A i novel andimproved diusion cloud chamber having rnag-` A nification of thesensitive zone, a strong continuous light-`,V n

ing which does not appreciably affect the temperature,

and a cavity arrangement for dark background viewing. l Another featureof the invention has been thevprovi- .L sion of a novel and improveddifusion cloud chamber.'

which has no troughs which occlude any portion of the field of view.

Still another feature of the invention has been the provision of a noveland improved diffusion cloud chaml ber in which vapor is suppliedfrom-absorbent material situated on the sidewalls of the optical tube. YY

Another feature-'of the inventionY has been the pro- Patented` Mar. 28,'1961 vision of a novel and. improved diusion cloud chamber in whichdewfall on the eyepiece is avoided.

Other and further objects, features and advantages of the invention willappear more fully from the followingi description of a preferredembodiment of the invention which is illustrated in the drawing.

The drawing is a longitudinal sectional view of a preferred diiusioncloud chamber in accordance with the invention.

Referring now to the drawing, the ditusion cloud adversely affect theoperation of a cloud chamber. Thedesign of the illustrated form of theinvention, both with respect to shape and size of parts, represents whatis believed to be an optimum combination. So far as is presently known,the size and shape of the various parts,

particularly the cloud chamber space and viewer assembly, are criticalfor securing best results. For this reason, the drawing illustrates thecloud chamber of the invention substantially in accurate full scale.

The cold reservoir is illustrated as an open-necked insulating bottle 14having a hollow chamber 15 adapted to be iilled with a freezing mixture.The bottle 14 is preferably of the Dewar liask type and may be acommercially available thermos bottle. For convenience the internalstructure of the bottle 14 has not been illustrated. In one form of theinvention, the bottle 14 was an Aladdin one pint thermos bottle. Thecharge in the chamber 15 should be one which will produce a temperatureof -50 F. or lower. A variety of freezing mixtures are suitable for thispurpose, for example, white gasoline stirred with small lumps of DryIce. A desirable freezing mixture, because of its odorless andnon-toxiccharacteristics, is Freon liquefied among lumps of Dry` Ice.

The cold transfer assembly 11 comprises a cold transfer rod 16, a hollowcylindrical stopper 17 and a washer 18. The stopper 17 has a side wall19 shaped to t within and close the open neck of the bottle 14 and abottom wall 20 having a central hole adapted to accommodate centralcylindrical portion 21 of the rod 16. The rod 16 has a peripheral groove22 which is engaged by the rim of the hole in the bottom wall 20 therebyto retain the rod 16 and stopper 17 in the relative positionsillustrated.

The end of the rod 16 within the chamber 15 is tapered, as shown at 23,making an angleof about 20 with the horizontal. The central portion 21of the rod 16 terminates at a shoulder 24, the remainder of the rod 16having a larger diameter, as shown at 25. The end 25 of the rod 16 isprovided with a bore forming a chamber 26 which may be about 1 inch indiameter and about 1.125 inches in length, exclusive of the tapered end27. A cylinder 28 of thin black material is provided in the chamber 26.A disc 29 of similar material is provided in the inside end of thechamber 26. The cylinder 28 and disc 29, which may be made from a singlethickness of black art paper, form a lining for the chamber 26.

The rod 16 should be made from a highly heat conductive material such asaluminum. The stopper 17 may be made from rubber or a low temperatureresistant plastic.

`The washer 18, which may conveniently be made from cork, rubber or alow temperature resistant plastic, tits tightly between the bottom 20 ofthe stopper 17 and the shoulder 24, the washer being in intimate contactwith the adjacent wall of the rod 16. The diameter of the washer 18 isslightly greater than the diameter of the end portion 25 of the rod 16.The bottom 20 of the stopper 17 may be provided with a small hole, ifdesired.

The Dewar ask 12 is open at both ends so as to 4 form a hollow cylinder.The walls of the Dewar aslc should be made from a temperature changeresistant glass such as Pyrex glass. The outer wall 30 might be, forexample, 55 mm. thick while the inner wall 31 might be 38 mm. thick, thespace between being evacuated through a sealed-off opening 32. One end33 of Dewar ask 12 fits between theouter surface of the washer 18 andthe inner surface of the stopper 17. A sealing contact is affordedbetween the wall 31 and the washer 18.

The eyepiece and viewer assembly 13 comprise a bush-'- ing 34, a hollowcylinder 35 and a glass lens 36 forming an eyepiece.

The bushing 34 its within end 37 of the Dewar flask 12 and is cementedthereto for retaining the bushing in the end of the iiask 12 and forproviding an air-tight lit. An outwardly extending ange 38 on thebushing 34 contacts the end of the ask 12. The bushing 34, which may bemade of aluminum, is provided with internal threads which are inthreaded engagement withmade in the form of a resilient washer, such ascork,y

fitting tightly in the end of the ask 12 and permitting sliding motionof the cylinder 35. In such case threads -would not be provided on thecylinder 35.

Lens 36 is cemented in place in a shoulder 41 provided in the outer endof the cylinder 35. The cylinder 35 is provided with a threaded holeadapted to receive a removable screw 42. Theinner surface of thecylinder "35 is provided with a lining 43 of absorbent material such asseveral layers of filter paper. The liningl 43 may be held in placeagainst relative axial movement by a circular spring 44 acting in agroove provided in the inner wall of the cylinder 35. In the embodimentof -the invention illustrated, the internal diameter of the cylinder 35is .875 inch while the axial length thereof is 2.375 inches.

To put the cloud chamber into operation, the subassembly formed by theelements 11, 12 and 13 is removed from the cold reservoir 10 and thechamber 15 of the latter is'tilled with the freezing mixture, e.g.

Freon4 and Dry Ice. The subassembly is then reinserted' into theposition shown. With a freezing mixture as described, the stopper 17should be disposed at a point at which carbon dioxide gas evaporatedwithin the chamber 15 is allowed to escape.

The filter paper 43 is wet with the supersaturating vapor source,preferably methanol, either by removing the screw 42 and dropping theliquid on the paper or Vby removing the cylinder 35 from the bushing 34,wetting the paper, and -reinserting the cylinder 35. The space withinthe cylinder 35, the chamber 40 and the chamber 26 are {illed with vaporevaporated from the. kfilter paper 43. The temperature within thechamber 40 will decrease to the desired value for vapor condensation byheat conduction through the rod 16, this temperature being somewhatabove the temperature of the freezing mixture. Actually a substantialtemperature gradient will exist within the chamber 40 because one sidethereof is exposed to the cooling effect of the rod 16 while the otherside is exposed to the warming effect of the room temperatures. Theatmosphere within the chamber 40A will be supersaturatedwithmethanolvapor.

Vapors other than methanol-can bue'used, of course, but

they preferably have comparable volatlities, for example, mixtures ofmethyl and ethyl alcohol and water.

Within the region of supersaturation, i.e., within the chamber 40,tracks will be produced whenever a charged `particle passes through. Bypassing a strong beam of light laterally through the chamber-40, asshown byl the` dotted arrow 45, these tracks are made visible and can beviewed, either directly or by photographie methods, through the eyepiece13. Proper focus yfor the lens 36 can be secured through rotatingthecylinder 35. Motion picture records of the cloud trails have provenvery instructive.

Cloud chamber operation can be improved by providing an electric fieldacross the chamber y40, for example, by connecting the rod 16 and thebushing 34 to a source of high A.C. o-r D.C. voltage, e.g., 1000 volts.

While the cloud chamber can be operated with atmospheric air in thechamber 40, in many cases it will be desirable to replace the air withsome otherv atmosphere. This may be done by inserting a tube in place ofthe screw 42, pumping out 'the airlfa'nd refilling with the desiredatmosphere. Track length may be increased by filling with low densitygases or decreased by filling with higher density gases. f

As illustrated, the emergent portion of the 'eyepiece holder (cylinder35) is exposed to ambient temperature and it is this temperature whicheffectively controls the delivery of methanol vapor from the lter paper43. The delivery of methanol vapor may be limited by surrounding theemergent portion of the cylinder 35 with a thermally insulating wrapper.On the other hand, such delivery may be increased by subjecting theemergent portion of the cylinder 35 to a heat source, such as anincandescent bulb.

Condensed methanol may be removed either by removing the eyepiece holderor the screw 42 and inverting the subassembly of elements 11, 12 and 13.

While the invention has been described in connection with a specificembodiment thereof, various modifications Will occur to tho-se skilledin the art without departing from the spirit and scope of the inventionas set forth in the appended claims. However, as explained previously,it is believed that the best performance will be obtained with a cloudchamber constructed and proportioned as illustrated.

What is claimed is:

l. A diffusion cloud chamber, comprising a hollow heat insulating memberopen at both ends, a cold reservoir, a heat conductive member having oneend thereof extending into said cold reservoir and having the other endthereof extending into one end of said heat insulating member, mountingmeans for supporting said heat conductive member in said one end of saidheat insulating member, viewing means mounted in the other end of saidheat insulating member, and having an emergent portion disposed outsidesaid heat insulating member, the adjacent ends of said heat conductivemember and said viewing means being spaced within said heat insulatingmember to define a chamber, and a source of a volatile liquid incommunication with said chamber, said of said heat insulating member forevaporatin'g` said heat insulating member being yconstructed so thatsaid chamber may be illuminated externally whereby cloud trails createdwhen ionizing particles pass through said chamber may 'be viewed throughsaid viewing means.

2. A diffusion cloud chamber, 'comprising a hollow generally cylindricalheat insulating -member open at both ends, a cold reservoir, a heatconductive member having one end thereof extending into said coldreservoir and having the other end thereof extending into one end of`said -heat insulating member, mounting means for supporting said heatVconductive member in said one end of said heat insulating member andfor supporting said one end of said heat insulating member in said coldreservoir, viewing means comprising a highly heat conductingIcylindrical tube having one end thereof mounted in the other end ofsaid heat insulating member and having an emergent end disposed outsidesaid heat insulating member, the adjacent ends of said heat conductivemember and said Viewing means being spaced within said heat insulatingmember to define a chamber, and a source of a volatile liquid incommunication with said chamber, said source being located within saidtube and volatile liquid to provide -super'saturated vapor in said`chamber, said heat insulating member being constructed .so that saidchamber may be illuminated externally whereby cloud trails created whenionizing particles pass through s'aid supersaturated vapor in saidchambermay be viewed through said viewing means.

3. A diffusion cloud chamber, comprising a cylindrical Dewar flask openat both ends, 'an insulated container open at one end and adapted tocontain a freezing mix ture, a metallic heat conductive rod, mountingmeans for supporting said rod at an intermediate portion thereof in oneend of said Dewar flask, stopper means for closing said open end of saidcontainer and for 'supporting said one end of said Dewar flaskin saidopen end of said container whereby one end of said rod extends into saidcontainer, the other end of said rod extending into the interior of saidDewar flask, a hollow cylinder open at one end and having an opticalelement mounted in and closing the other end thereof, means lto mountsaid cylinderrin the other end of said Dewar flask with said open end ofsaidcylinder extending into the interior of said Dewar flask, theinterior of said Dewarflask between the adjacent ends of said'rod andsaid cylinder forming ay chamber, and a layer of absorbent material inthe inside of said cylinder, said material being adapted to serve as asource of volatile vapor for providing a supersaturated vapor in saidchamber, the walls of said Dewar flask, at least adjacent said chamber,being transparent to afford a path for external light to pass throughsaid chamber whereby cloud trails created when ionizing particles passthrough said chamber may be viewed through said optical element. v

4. A diffusion cloud chamber, comprising a cylindrical Dewar flask openat both ends, an insulated container open at one end and adapted tocontain a freezing mixture, a metallic heat conductive rod, mountingmeans for supporting said rod at an intermediate portion thereof in oneend of said Dewar flask, stopper means for closing said open end of saidcontainer and for supporting said one end of said Dewar flask in saidopen end of said container whereby one end of said rod extends into saidcontainer to provide substantial -heat transfer contact between-said rodand said freezing mixture, the other end of said rod extending into theinterior of said Dewar flask, a hollow cylinder Vopen at one end andhaving an optical element mounted in and closing the other end thereof,means to mount said cylinder in the othervend of said Dewar flask withsaid open end of said cylinder extending into lthe interior of saidDewar flask,l the interior of said Dewar flask between the adjacent endsof said rod 'and said cylinder forming a chamber, and a layer ofyabsorbent material in the inside of said cylinder, said material beingadapted to serve as a source of volatile vapor for providing asupersaturated vapor in said chamber, vaporization of said volatilevapor occurring through heat transfer between said material andatmospheric air external of saidy cylinder, the walls of said Dewarflask, at least adjacent said chamber, being transparent'to afford apath for external light to pass through said chamber whereby cloudtrails created when ionizing particles pass through said chamber may beviewed through said optical element.

5. A diffusion cloud chamber, comprising a cylindrical Dewar flask openat both ends, an insulated container open at one end and adapted tocontain a freezing mixture, a metallic heat conductive rod, mountingmeans for v supporting said rod at an intermediate portion thereof inone end of said Dewar flask, stopper means for closing said open end ofsaid container and for supporting said one end of said Dewar flask insaid open end of said container whereby one end of said rod extends intosaid container to provide substantial heat transfer contact between saidrod and said freezing mixture, the other end of said rod extending intothe interior of said Dewar flask,

element mounted in and closing the other end thereof,

means to mount said cylinder in the other end of said Dewar flask withsaid open end of said cylinder extending into the interior of said Dewarflask, the interior of said Dewar flask between the adjacent ends ofsaid rod and said cylinder forming a first chamber, said other end ofsaid rod having an opening forming a second chamber communicating withsaid first chamber and in line of sight with said optical element, saidsecond chamber having a black lining, and a layer of absorbent materialin the inside of said cylinder, said material being adapted to serve asa source of volatile vapor for providing a supersaturated vapor in saidfirst chamber, the walls of said Dewar flask, at least adjacent saidfirst chamber, being transparent to afford a path for `external light topass through said first chamber whereby cloud trails created whenionizing particles pass through said first chamber may be viewed throughsaid optical element.

6. A diffusion cloud chamber, comprising a cylindrical Dewar flask openat both ends, an insulated container open at one end and adapted tocontain a freezing mixture, a metallic heat conductive rod, mountingmeans comprising a washer for supporting said rod at an ntermediateportion thereof in one end of said Dewar ask, stopper means for closingsaid open end of said container and arranged to cooperate with saidwasher for supporting said one end of said Dewar flask in said open endof said container whereby one end of said rod extends into saidcontainer to a point adjacent the bottom of said container, the otherend of said rod extending into the interior of said Dewar flask, ahollow cylinder open at one end and having an optical element mounted inand closing the other end thereof, means to mount said cylinder in theother end of said Dewar flask with said open end of said cylinderextending into the interior of said Dewar flask, the interior of saidDewar flask between the adjacent ends of said rod and said cylinderforming a first chamber, said other end of said rod having an openingforming a second cylindrical chamber in communication with said firstchamber, said second chamber having a diameter substantially equal tothe diameter of said cylinder and being in axial alignment therewith,said second chamber having a black lining, and a layer of absorbentmaterial disposed inside of said cylinder, said material being adaptedto serve as a source of volatile vapor for providing a supersaturatedvapor in said first chamber, the walls of said Dewar flask, at leastadjacent said first chamber, being transparent to afford a path forexternal light to pass through said chamber whereby cloud trails createdwhen ionizing particles pass through said first chamber may be viewedthrough said optical element.

7. A diffusion cloud chamber, comprising a cylindrical Dewar flask openat both ends, an insulated container open at one end and adapted tocontain a freezing mixture, a metallic heat conductive rod, mountingmeans for supporting said rod at an intermediate portion thereof in oneend of said Dewar flask, stopper means for closing said open end of saidcontainer and for supporting said one end of said Dewar flask in saidopen end of said container whereby one end of said rod extends into saidcontainer, the other end of said rod `extending into the said cylinderforming a first chamber, said other end of said rod having an openingforming a second cylindrical chamber in communication with said firstchamber, said second chamber having a diameter substantially equal tothe diameter of said cylinder and being in axial alignment therewith,said second chamber having a. black lining, and a layer of absorbentmaterial in the inside of said cylinder, said material being adapted toserve as a source of volatile vapor for providing a supersaturated vaporin said first chamber, the walls of said Dewar flask, at least adjacentsaid first chamber, being transparent to afford a path for externallight to pass through said first chamber whereby cloud trails createdwhen ionizing particles pass through said first chamber may be viewedthrough said optical element.

8. A diffusion cloud chamber, comprising a cylindrical Dewar flask openat both ends, a vacuum insulating container open at one end and adaptedto contain a freezing mixture, a metallic heat conductive rod, mountingmeans comprising a washer for supporting said rod at an intermediateportion thereof in one end of said Dewar flask, stopper means forclosing said open end of said container and arranged to cooperate withsaid washer for supporting said one end of said Dewar flask in said openend of said container whereby one end of said rod extends into saidcontainer, the other end of said rod extending into the interior of saidDewar flask, a highly heat conducting hollow metal cylinder open at oneend and having an optical element mounted in and closing the other endthereof, said cylinder having external threads adjacent said one endthereof, means to mount said cylinder in the other end of said Dewarflask with said open end of said cylinder extending into the interior ofsaid Dewar flask, said last mentioned means comprising a bushing infixed sealing engagement with said other end of said Dewar flask andhaving internal threads in threaded engagement with said externalthreads, the interior of said Dewar flask between the adjacent ends ofsaid rod and said cylinder forming a first chamber, said other end ofsaid rod having an opening forming a second cylindrical chamber incommunication with said first chamber, said second chamber having adiameter substantially equal to the diameter of said cylinder and beingin axial alignment therewith, said second chamber having a black lining,and a layer of absorbent material in the inside of said cylinder, saidmaterial being adapted to serve as a source of volatile vapor forproviding a supersaturated vapor in said first chamber, the walls ofsaid Dewar flask, at least adjacent said first chamber, beingtransparent to afford a path for external light to pass through saidfirst chamber whereby cloud trails created when ionizing particles passthrough said rst chamber may be viewed through said optical element.

9. A diusion cloud chamber as set forth in claim 8 in which saidcylinder has an opening in the wall thereof, and removable means forclosing said opening.

10. A diffusion cloud chamber as set forth in claim 3 in which meansyare provided to create a high potential difference between said rod andsaid cylinder.

1l. A diffusion cloud chamber as set forth in claim 8 in which saidblack lining is made from a black paper cylinder having an open endfacing said first chamber and having a black paper end.

12. A diffusion cloud chamber as set forth in claim 8 in which saidabsorbent material is filter paper.

References Cited in the file of this patent Snowden: The Diffusion CloudChamber, Progress in Nuclear Physics, vol. 3, pages 1 to 17, AcademicPress, New York, 1953.

Fretter: Nnclear Particle Detection (Cloud Chambers and BubbleChambers), Annual Review of Nuclear Science, vol. 5, pages -167 (1955),Annual Reviews, Stanford, Calif.

